Ion acceleration system



Feb. 23, 1960 J. 5. LUCE arm. 2,926,251

ION ACCELERATION SYSTEM Filed July 18, 1956 2 Sheets-Sheet 1 I I n IIIIIIII I I I v P- um 4 24 f:

IN VEN TORS BY John S. Luce and John A. Marfin ATTORNEY Feb. 23,, 1960J. 5. LUCE ETAL ION ACCELERATION SYSTEM 2 Sheets-Sheet 2 Filed July 18,1956 IN VEN TORS v BY John S. Luce and 7 John A. Mari-In Maya/4MATTORNEY United States Patent ION ACCELERATION SYSTEM John S. Luce andJohn A. Martin, Oak Ridge, Tenn.,

assignors to the United States of America as represented by the UnitedStates Atomic Energy Commission Application July 18, 1956, Serial No.598,725 7 Claims. (Cl. 250-413) The present invention relates toproduction of relatively large beams of positive ions, and moreespecially to a method of and apparatus for forming intense beams ofpositive ions including an ion source and means to accelerate ions fromsaid source through grid electrodes to a desired region of focus. Oneform of the apparatus may be incorporated with conventional'structure toform an improved embodiment of the isotopeseparator called the calutron.

The calutron is a device for accomplishing isotope enrichment or massseparation of positive ions by accelerating them through an intensemagnetic field between a source and a collector. In the calutron, asolid sample material is vaporized by heating, the vapor is ionized byelectron bombardment inside an ion source which is maintained at a highpositive potential, ions are pulled out of the source by ahighlynegative accelerating electrode disposedadjacent the mouth of thesource, the ions passing through the accelerating electrode aredecelerated by causing them to pass through a second grounded electrode,and the resulting beamof ions is passed through the intense magneticfield to the collector. A complete description of a suitableembodimentof the calutron is contained in US. Patent 2,709,222, issuedto 2,926,251Patented Feb. 23, 19 0 point is termed the saddle point. It was believedthat positive electric fields penetrated the negative accelerating slit,and accordingly a flat grid was placed in the accelerating slit tostraighten the positive fields. As a result, the focal region was movedto the grid openings and the beam diverged badly, so that unsatisfactoryfocusing of the beams on the collectors resulted. Other studiessuggested that the beam might be focused if the arc plasma were bowedconvex, rather than concave, as if a virtual focus point existed behindthe are, but such focussing was never achieved. Widely divergent beamsresultedfrom operation with a single focussing electrode, because theelectric field was so weak in thecenter that .the arc bowed toward theaccelerating electrode. With such divergent beams, increasing arccurrent or voltage only increased the numbers of ions draining to theelectrode.

With a knowledge of the shortcomings of the known methods and apparatusfor obtaining well-focused, intense ionbeams, applicants have, as aprimary objectof their invention, provision of a method of and means forobtaining a relative intense, well-focused ion beam. Another object ofthe invention is to stabilize the arcplasma in an ion source whilesimultaneously withdrawing an ion current of relatively high intensitytherefrom. ,A further objectof the invention is to provide a generatorof positive ions including an ion source provided With.a grid-like ionexit aperture forming a plurality of small, discrete beams, and anaccelerating grid disposed closely adjacent the source to further focusand define the discrete beams. Another objectis. to provide awell-focused intense beam of ions with a single accelerating electrodeand a single voltage source. .Yet another object of the invention is toprovide means for obtaining large ion beam by means of an ion source andan accelerating grid structure which has a small convex curvature toobtain greatly improved focusing of the ion beam at the collector. Themajor object of the invention is to eliminate the disadvantages inherentin ion beam generators having a narrow focal region in front of thesource by providing novel means for withdrawing the beam from the ionsource and for focusing it in such a manner that all the beam in theacceleration region, resulting from the coulomb repulsion accompanyinglarge beams of charged particles. he observed calutronperformanceis in:aceord with theory, sinceapplication of .Ghilds 3/ 2-power lawindicatesthat no mo re than .about 46 ma/in. of emission couldbe-expectedto pass through a focal: point. In, prior calutronoperation,a positive-ion sheath forms between the arc plasma and the acceleratingelectrodes, the boundary between the arc plasma and the sheath beingknown as the center of the meniscus. The meniscus is pushed back toward,the arc plasma, away from the source grid by the accelerating fieldwhich extends into l-the ,iorieiiit slit region and presents a concavesurfacetoward the arc plasma, so thatall ions leaving the me sens normalto thesurface thereof must passthorugh narrow ionexinslit in thesourceand a small focal region the vicinity of the accelerating electrode. Thefocuss ing effect of the dimple in the arc plasma as caused 'bythe'meniscus being pushedback as discussed above results in excellentseparation factors for different isotopes collected at the "180position.

In prior calutronstudies, a'region of ionization due-to oscillatoryelectrons was notice in the accelerating elec trode slit which causedthe ion bearnto pass'thro'ugh a focal point in the acceleratingelectrode slit and ions no longer must pass through any small focalregions.

These ,objectsare attained'by providing a series of small apertures inthe front'face of the ion source, rather than the one continuous ionexit slit, and a single corresponding grid spaced very .closely adjacentthe ,ion sourceto replaceboth. the prior accelerating and deceleratingelectrodes. With the sourceata positive potential, the accelerating gridmay be grounded. With such arrangement,.intense normally emergent ionbeams maybe 1 p'roducedifrom the source and focussed at a desiredlocation without first passing through a common focal point, so that nobeam blow up occurs due to coulomb repulsion. One or moreofthe grids maybe convex relative to the front face of-the ion source, to improve thefocus of the beam ,at .the desired location. By providing a grid acrossthe ion-exit slit, the are is strengthened in the center and shieldedfrom external fields, so that relatively large currents may be obtainedwith relatively low accelerating potentials. Preferably, theaccelerating grid is curved to improve the focus, while the source gridis .flat, tending to push'back the center of the arc and therebypreserve the focus,

;Means by which the above objects may be attained are illustrated in theappended drawings, wherein: Figure 1 illustrates schematically one formof my improved 'fo lsystem;

Figure 3 illustrates a partial plan view of one form of an acceleratinggrid;

Figure 4 illustrates a partial plan view of one form of a correspondingsource grid;

Figure 5 illustrates a preferred embodiment of the grid focal system asit is utilized for isotope separation.

Figure 6 illustrates a partial plan view of the accelerating grid ofFig. 5;

Figure 7 illustrates a partial plan view of the source grid of Figure 5,and

Figure 8 shows schematically a flat-grid source used to inject ions intoa system. Referring now to Fig. 2, a first grid focal system is shownincorporated in a conventional calutron ion source box, including arectangular graphite box 20 defining an arc chamber 21 therewithin. Onewall 22 of the box is provided with a defining slot 24 through whichelectrons are accelerated from a filament mounted adjacent the wall 22,across the arc chamber, through an aperture 25, to an anode mountedexternally of the wall 23. A suitable source box is shown in my priorPatent No. 2,700,107, issued January 18, 1955. A potential of 50-300volts between box 20 and the filament may be provided to accelerateelectrons into the chamber. A vapor-entry slot in the bottom of the boxreceives vapor as described in the patents cited above. Electrode 26bridges the vapor exit slit in the top wall of the box, and may beprovided with a plurality of aligned small aper- 'tures 29 as shown inthe plan view, Figure 4. The source grid 26 may be a separate gridstructure placed in a corresponding aperture in the top wall of the box20, as shown in Fig. 2, or it may be a separate plate as shown in Fig.4, provided with a grid like portion and also a portion withoutapertures, such that it may be fastened to the holds the box 20, througha suitable insulator. The spacing between grids is made very small, butmay vary from about ,4 to going from mass 1 to mass 235. Onesatisfactory source comprised a grid 27, A thick at its outer edges, andtapering to 4," thick in the center, grid-like portion, with a grid 26only & in thickness. In one suitable arrangement, the apertures in thesource grid were x while the slots were ,4 apart along the line of theirshortest dimension and apart along the line of their longest dimension.Corresponding apertures in the accelerating grid were A x 7 and theapertures were spaced apart along the line of their shortest dimensionand apart along the line of their longest dimension. From the aboveexemplary dimensions it may be noted that the apertures in the grid 26are slightly shorter than those of the grid 27, and that the wallsbetween the apertures in grid 27 are correspondingly thinner to allowfor alignment of apertures in the two grids.

Referring now to Figure 1, an improved embodiment of the grid focusingsystem is illustrated schematically. Rectangular graphite box 1 definesan arc chamber 2 in which is established an arc plasma 3 comprisingpositive ions and electons. A flat source grid 4 forms the front wall ofthe arc chamber and is provided with a plurality of apertures to dividethe ion beam into a plurality of discrete, small beams. The grid ismaintained at the same potential as the arc chamber box 1. Spacedclosely adjacent the grid 4 is an accelerating grid 5, the central gridportion of which is slightly convex in curvature. A negativeaccelerating potential is applied between the grid 5. and the box 1 toform an ion beam and to accelerate it into the analyzing field along thepaths illustrated into a focus along the focal line 6, where a collectormay be positioned. It will be noted that the focal line is no longer 180from the arc chamber or from the accelerating region as it is in thenormal calutron operation. Rather the curvature of the accelerating gridcauses ions of like mass-to-charge ratio to be focused at a point lessthan 180 from the accelerating region, as shown. This focusingarrangement may be better understood by considering that a virtual focusis formed at a point behind the arc chamber box 1 along the line 7,displaced 180 from the focal line 6. It may be seen that by virtue ofthis novel apparatus, the actual ion beam never goes through a realfocal point until it reaches the collector, so that there is not now aspace-charge imposed limitation on the amount or current density of thebeam going through a saddle point.

Figure 5 illustrates schematically how large ion currents are withdrawnfrom the arc plasma, accelerated into an analyzing field, separated intodiscrete beams of ions having different rnass-to-charge ratios, andcollected in suitable collector pockets. The substance to be separatedmay be vaporized in any conventional calutron furnace, such as thatshown in the Lawrence patent, supra, and allowed to enter an especiallyconstructed, one-piece graphite source box 31 through vapor entry slit32 in graphite plate 33 which defines the rear of the arc chamber 34.Rather than being provided with the conventional ion exit slit in theforward wall, source box 31 is provided with a slightly convex gridportion having a plurality of small, parallel slots therein. Figure 6illustrates the front view of the source box 31 looking into the gridportion. The slots 35 are drilled into the slightly convex portion 36,which may be machined in the same block of graphite forming box 31. Theelectron collimating slits, allowing entrance of the electron beam fromthe filament and exit of the electron beam to the anode are placed inline with the inner edge of the grid portion as indicated by theslot-shaped arc plasma 37, so that the arc will be formed adjacent thegrid portion, and just behind the slit 35.

The accelerating grid 38 may also be a unitary graphite structure inwhich is machined a grid portion 39 which is slightly concave in thedirection facing the convex grid 36. A front view of grid 38, lookinginto the grid from the left in Fig. 5, is shown in Fig. 7. A pluralityof parallel, aligned slots 40 are provided in the curved portion 39 ofthe grid, the front portion of the member 38 adjacent the slits beingcut away to form a recess 41 to allow ample clearance for the ion beam.The grid 38 may be held in place adjacent source 31 through aninsulating spacer member fastened to the same frame as is the source, orit may be held in place by a separate support or any other suitablemeans.

It has been found that the arc plasma is stabilized by the curved grid36 and that large quantities of ions may be withdrawn from the plasma,since there is no focal point in the accelerating region to limit thenumbers of ions which may pass therethrough. Ions may be collected insuitable collector boxes 42 provided with pockets 43, 44 for receivingions of different masses. It is to be noted that the collector 42 isdisposed forward of the source 31 rather than directly above it, as isusually the case because of the virtual focus method of operationachieved by the described curved grid system. With the systemillustrated, a broad, intense beam originates at the ion source andnarrows to a focus only when it arrives at the collector. Relatively lowaccelerating potentials are required for the accelerating electrode.

In a preferred embodiment of the apparatus of Figure 5 adapted forseparation of isotopes having respective mass to charge ratios of 7 and6, collection has been achieved with a separation of 1% inches at thecollector, the collector being 3 1 inches forward of the source box.

netic field of 3.100 gauss, .collection is readily achieved,

--although it willtbe ap arent'ithaLother*suitable fieldtintensities andzaccelerating voltages may heiprovided.

.By way ofrfurther illustration, :the slits :35 :may be PA inch inheight while :the :slits :40;may be 7% :inch in -:height. Those :slits Ymay be .inch :across and separated by spacer members A inch wide. thebars between the slits are preferably beveled *att45", the bars being' Ainch in thickness andzthe bevel extending approximately /2 :of this.thickness.. -.In'one embodiment-o'f an ion source used, thegrid memberswere 16% inches in'length, with 13 inches between center lines of thefirst and last of the parallel grids. Thegridsiin the cut-away .portionof member 38 may be inch in thickness, or twice the thickness of thegridspacer membersinthe source 31. While considerably largertotalioncurrents can be consistently received, it has been found thatthe beamfocus is sufliciently sharp to assure acceptable isotopic enrichment attotal currents of around .700'niilliamperes with uranium. Beam currentsof about ing 'vertical, rather than horizontal grids, beam disper- -Theoutward edg -Sci sion=vertically-is about i 2%, rather than 27%, allow-I ing 180 isotope separation;

In the preferred embodiment shown the -source grid outer surface ismachined to a 4 inch radius,the outer surface of-the accelerating gridis machined to a4l s inch-radius, and the inner surface of-theaccelerating grid is machined to a 4 inch radius. The cutawayportion ofthe accelerating'grid is 1% inches at its outermost point. t

It hasbeen found that replacing the high potentialarecelerating-electrodeof the calutron by the 'present grid,

closely adjacent the arc and maintained at the'same potential'as thearc.chamber, greatly-stabilizes the 'arc,"so that better operation of theentire unit results; It has been'further found thatwith this novelacceleration systemthe source may be operated at 20 kilovolts ratherthan at 38 kilovolts as required in'standard'ca-lutron operation, theaccelerating .grid may be operated at ground potential, and the'highnegative potential supply formerly used for the accelerating slit is nolonger required. Elimination of the requirement of thehigh negativevoltage thus eliminates a'costly power supply and also makes operationmore efficient, sincethe ion drainto the highly negative electrode iseliminated. Reduction the potential requirements of the acceleratingvoltage source*from "38' to 2()kilovolts achieves a further major costsavings in both original cost and operating expense.

Further, as pointed'out above, the ratio of power supply drain to theuseful ion beam, which forms an index to the cost of operation, isappreciably smaller, that is 1.3- 2.5, as compared with 4.5 for aconventional calutron source.

It has further been found that sources of the character described hereinare highly useful as sources of large currents of ions for injectioninto a circulating charged particle system. Since the ions of the beamemerging from the flat grids of the source come to a focus at the90-degree point, such point is a convenient point of entry for the ionsinto a circulating system.

As shown in Fig. 8, ions are formed in a source chamber defined by thewalls of box 51 by means of an electron beam passing from an externalfilament to the box wall or to a cathode. The are discharge 52 so formedis shown in cross-section. A flat source grid 53 is formed by verticalslots in the front face of box 51, corresponding to the grid shown inFig. 7, except that grid 53 is parallel with the rear wall 54 of box 51.A corresponding flat accelerating grid 55 is provided closely adjacentgrid 53 in the same closely-spaced relationshipas between the curvedgrids of Fig. 5. Ions from the source are accelerated by a potentialapplied between the grids, and are brought to a focus at the 90 point bythe applied magnetic field, which is shown by the dot H as coming out ofthe plane ofthe paper. The broad 1 focused. The long dimension of thegrids may bejeither vertical orhorizontal, that is, normal or parallelto the magnetic field. It has been found that better focused beams maythe .obtained, for purposes such as isotope separation, by usingvertical grids. The individual ion beams-.emetgingfrom the slits appearto repel each other so=as.to:reduce.divergence,parallel to the magneticfield, providing .a-narrower, more uniform beam than can he obtained byhorizontal grids, where there is not that unifying effect bunchingthebeams together.

It :may .be 1seen that' the novel means for generating ions hereindescribed-may be applied in various ways to differentproblemssuch asinjection of ions into accelerators,wisotopetseparations, and the like.Otherapplications whereintense beams of singly and multiply charged ionswill becapparent torthose skilled in the art. 'It is,therefore,-intended that the scope of the invention not be .1limited tothe physical embodiments described, but only by the attached claims.

Having {described :the invention what is claimed as novel is:

:1. .In isotopeseparation equipment comprising an ion .sourcer-having.an ion exit passage, an ion collector, means forr-providingaastrongmagnetic field between said source and collec.tor,.-;and1a sourceof potential for ac- .celeratingionsyfrom,said ion source to saidcollector through said field, the improvement comprising a firstmultitapertured plane .;.member forming a grid disposed acrosssaid exitaperture and contacting the Walls of said source, :a second'multi-apertured convex member forming a-focussinggrid disposed inspaced relation-to said firstgridzbetween saidsource and collector, saidgrid aperturesrhaving.their-longest dimension normal to the direction.of said magnetic field, and means connecting said-source ;ofaccelerating potential between said first and saidsecond grids.

-2. In..-isotope separating means provided with an are chamber, a sourceof -.ions, means, for establishing an arc withinas'aidrchamber,.one:wall of said. chamber being provided with an aperture for theescape of ions, an ion collector disposed in spaced relationships fromsaid source, and means for establishing a strong magnetic field betweensaid collector and said chamber, the improvement comprising a firstplane grid member disposed across said aperture and contacting the wallsof said chamber, a convex grid member disposed adjacent said plane gridmember between said chamber and said collector, said grids havingopenings the longest dimensions of which are normal to the direction ofsaid magnetic field, and a source of potential coupled to said chamberand convex grid to accelerate ions to said collector, the radius ofcurvature of said convex grid member, the potential established by saidsource, and the strength of said magnetic field being so related thations of a selected mass from said chamber pass through said grids andcome to a focus upon said collector.

3. In ion generating means of the character described including an ionsource provided with an ion exit aperture, an ion collector, and meansfor establishing a magnetic field between said source and saidcollector, the improvement comprising a first convex grid memberdisposed across said ion exit aperture, a second convex grid memberdisposed adjacent said first grid member and aligned between said firstgrid and said collector, said grid membershaving openings the longestdimensions of which are normal to the direction of said magnetic field,both of said grid members having radii of curvatures operative to focusions of selected masses from said 7 source upon said collector, andmeans coupled between said grids for accelerating positive ions fromsaid source through said grids to said collector. I

4. In an ion producer comprising means defining an arc chamber having anion exitaperture, means for establishing an arc therethrough, means forfeeding vapor into said arc, an ion receiving means disposed in spacedrelationship to said chamber, means for establishing a strong magneticfield encompassing said chamber and receiving means, the improvementcomprising; means for stabilizing said arc within said chamber, saidmeans comprising a first m-ulti-apertured grid member disposed acrosssaid ion exit aperture and contacting said arc; means for acceleratingions from said are through said field to said ions receiving meanscomprising a second multi-apertured convex grid member disposed closelyadjacent said first grid and aligned with the apertures therein; and asource of potential having a positive terminal connected to said arechamber and a negative terminal connected to said second grid toaccelerate positive ions from said chamber through said grid to saidcollector, at least said second grid having a radius of curvature suchthat ions of a selected mass from said source passing therethrough willbe focused upon said collector, said grid apertures having their longestdimension normal to said magnetic field and to the direction of iontravel therethrough.

5. In isotope separating means provided with a source of gas to beionized, said gas including at least two isotopes to be separated, anevacuated tank, means for establishing a magnetic field in said tank,and electron beam means for ionizing said gas within a chamber, theimprovement comprising a back plate having a central aperture forreceiving gas from said source; an elongated graphite block providedwith a generally U-shaped' cavity defining an ionization chamber andhaving a back surface contacting said back plate to receive gastherethrough, the ends of said block being provided with curved slitsconforming to the bottom of said cavity to allow passage of saidelectron beam therethrough, the bottom of said cavity and front surfaceof said block being correspondingly arcuate and convex and provided witha plurality of spaced slits; and an accelerating electrode provided witha corresponding arcuate, convex apertured portion disposed closelyadjacent said front surface of said block, a plurality of collectors,means connected to said front surface and accelerating electrode foraccelerating ions from said electron beam through said 'field to saidcollector, said collectors being disposed forward of said ionizationchamber at substantially from the virtual focal point defined by saidarcuate surfaces and said front surface slits and electrode apertureshaving their longest dimension normal to the direction of said magneticfield.

6. In a source of ions for particle accelerators and the like, providedwith a source box defining an ionization chamber, means for admittingvapor to be ionized to said chamber, and means for establishing an arcdischarge across said chamber, the improvement comprising: means forestablishing a magnetic field parallel to said arc, means to receivesaid ions disposed in said field in spaced relation to said box, a firstplanar ion exit grid electrode provided with a plurality of parallelapertures, a second planar grid electrode provided with correspondingapertures aligned with said first grid apertures disposed adjacent saidfirst grid, and means for establishing an electrical field between saidelectrodes to accelerate ions from said arc, said apertures beingaligned normal to the direction of said field and to a line drawnbetween said electrodes. 7 In an ion producer operating in a magneticfield and comprising an arc chamber provided with an ion exit aperturein one wall and means for establishing an arc discharge adjacent saidwall and parallel to said wall and said magnetic field to produce ions,the improvement comprising a first multi-apertured grid disposed withinsaid exit aperture in said wall to shield said arcfrorn externalelectrical fields, a second multi-apertured grid disposed closelyadjacent said first grid and outside said chamber, said apertures ofboth said grids being elongated with their longest dimension normal tosaid magnetic field and being aligned to permit ion egress, and a sourceof accelerating potential connected between said arc chamber and saidsecond grid to accelerate positive ions out of said chamber.

References Cited in the file of this patent UNITED STATES PATENTS2,624,845 Thompson Ian. 6, 1953 2,732,500 McLaren et al Jan. 24, 19562,743,370 McLaren et al Apr. 24, 1956 2,774,882 Wells Dec. 18, 19562,785,311 Lawrence Mar. 12, 1957

